1. Field of the Invention
This invention relates, in general, to electrical apparatus and, more specifically, to instrument current transformers, such as current transformers.
2. Description of the Prior Art
Instrument transformers, such as current transformers, are typically encapsulated in a thin layer of insulating material, such as butyl rubber or epoxy resin, in order to provide a weathertight seal around the current transformer. In encapsulating a current transformer, the assembled transformer is first placed in a suitable mold. The encapsulating material is then poured into the mold to fill all the cavities therein before being cured to a solid state. Although such encapsulating compounds provide satisfactory results, the encapsulating process is time consuming since it normally requires several hours to cure the encapsulating compounds, such as butyl rubber or epoxy resin, to a solid state. In addition, these types of encapsulating materials have become quite expensive.
It has been proposed to injection mold a layer of encapsulating material around electrical apparatus, such as current transformers. Such a process provides manufacturing advantages since the encapsulating materials can be hardened to a solid state in a matter of 2 to 3 minutes instead of the several hours associated with other types of molding materials. In an injection molding process, the encapsulating material is injected into a suitable mold containing the apparatus under extremely high pressures and temperatures. The high pressures involved in the injection molding process have heretofore prohibited the injection molding of an encapsulating material around current transformers. In a "through-type" current transformer having a secondary winding disposed in inductive relation with a toroidal-shaped magnetic core which has a central opening extending therethrough, the high pressures utilized in the injection molding process cause the shape of the magnetic core to be distorted and, further, cause the molding material to be extruded between the laminations of the magnetic core which affects the electrical performance of the transformer. In addition, it has been difficult to obtain an even coating of encapsulating material in the central opening of such current transformers. Various types of tubes or liners have been used within the central opening to provide insulation for the current transformer. However, it has been difficult to provide an adequate fluid-tight seal between such tubes or liners and the molding material. Mechanical seals, such as "O" rings, or adhesives have been used in the past; however, such means have proved to be costly and marginally effective in providing a fluid-tight seal between the liners and the molding material.
In current transformers of the type having primary and secondary windings concentrically disposed in inductive relation about a magnetic core, the problems involved in maintaining an adequate insulation space between the primary and secondary windings has prohibited the use of the injection molding process. In order to provide adequate dielectric strength between the primary and secondary coils, a certain minimum insulation clearance between the primary and secondary coils is required. However, due to manufacturing tolerances and the extremely high pressures used in the injection molding process, misalignment of the primary and secondary coils results which causes the major insulation clearance between the coils to be below the minimum required. As a result, an excess of insulation clearance would normally be designed into the current transformer. This causes the mean turn of the primary coil to be increased and results in a larger and more costly transformer.
Rod-like spacers have been utilized in prior art current transformers in order to prevent misalignment of the primary and secondary coils and thereby maintain a constant insulation clearance therebetween. However, the use of spacers made of standard insulating materials, such as fiberboard, cellulosic paper, fiberglass and "Micarta", do not adequately bond with the encapsulating material, which thereby results in an interface between the spacers and the encapsulating material which would form a short circuit path between the primary and secondary coils.
Thus, it would be desirable to provide an instrument transformer, such as a current transformer, suitable for encapsulation in an injection molded layer of insulating material. It would also be desirable to provide a current transformer in which the shape of a magnetic core is prevented from distorting under the high pressures involved in the injection molding process. It would also be desirable to provide a "through-type" current transformer wherein a fluid-tight seal is provided between a tubular liner in the central opening of the current transformer and the encapsulating material. It would also be desirable to provide the current transformer having primary and secondary windings in which the insulation clearance space between the primary and secondary windings is minimized and, further, is held constant despite the high injection molding pressures. It would also be desirable to provide a current transformer having spacer members disposed between the primary and secondary windings which are adequately bonded to the encapsulating material without an interface therebetween to eliminate short circuit paths between the primary and secondary winding.